Extensional flow mixer

ABSTRACT

An extension flow mixer especially for viscous liquids has a housing ( 10 ) with an end inlet ( 12   a   , 14   a ) connectable to a pressurized source of the liquid, and an outlet ( 119 ) at an opposite end of the housing ( 10 ). A mandrel ( 30 ) located in the cavity has protrusions ( 20′, 32′ ) with sloping side surface, the outer edges of which cooperate with the internal surface ( 114   a ) of the cavity to divide the cavity into a series of chambers separated by slits, such that liquid passes successively through all the chambers and slits in moving from the inlet ( 12   a,    14   a ) to the outlet ( 119 ). The slits have cross-sectional areas which decrease in the liquid flow direction. The mandrel ( 30 ) sides have helical grooves ( 134 ) forming passageways with the housing wall ( 114   a ) which allow liquid to be distributed evenly around the edges of the mandrel ( 30 ) to the inlet end or upstream chamber. The mandrel ( 30 ) may rotate to provide additional shear mixing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the mixing of liquids, particularlyviscous liquids, for example plastic materials such as polymers, andespecially the mixing of such materials having widely differentviscosities, and when a minor phase is highly viscous. However, theinvention can also be used for mixing other liquids, for example milkhomogenization and preparation of mayonnaise in the food industry,preparation of explosive emulsions in the explosive industry, andhomogenisation of molten soaps in the chemical industry.

2. Prior Art

One form of the present invention is an improvement of the motionlessextensional flow mixer described in our U.S. Pat. No. 5,451,106, issuedSep. 19, 1995, which gives a detailed review of the prior art in thisfield.

Briefly, it is known to mix polymers by distributive mixing effected byso-called “motionless mixers” placed between a screw feeder and a die.In most cases these mixers have a number of alternating right andleft-handed helical elements placed in a tubular housing equipped withtemperature control. The energy for mixing is provided by the pressureloss across the mixer. The splitting and recombination of streamsresults in a predictable number of striations. The advantage of suchmixers is that they are accessories to standard type of compounding orprocessing equipment, not their integral part, and their maindisadvantages are lack of easy adjustment, limited effectiveness inmixing, and inability to provide dispersive mixing. The basic principlebehind their design is division and recombination of the flow streams.Since the flow division is of the shear type, the dispersive forces areusually weak, limited to the cases where the two liquids show similarviscosity.

Theoretical calculations and experiments have shown that dispersivemixing of two Newtonian liquids is more efficient in extensional than inshear flow. Extensional flow occurs for example when fluid convergesfrom a reservoir to a capillary. In shear flow fields it is impossibleto disperse liquids that have viscosity higher than that of the matrixfluid by more than a factor of 3.8. By contrast, the dispersingcapability of the extensional flow field is only slightly affected bythe viscosity ratio. From the kinematics point of view, the extensionalflow field engenders deformation much more rapidly (note the absence ofvorticity in the elongational flow field). At a given stress level, thegenerated interphase (that is the accepted measure of adequacy of mixingor “mixedness”) is orders of magnitude greater than that generated inshear. Similarly, the amount of energy required to generate a givendegree of mixedness in elongation is orders of magnitude smaller thanthat in shear. Furthermore, the mechano-chemical degradation of themacromolecules is much less extensive in the elongational than in theshear field.

In spite of all these advantages present mixing equipment (including thetwin-screw extruders) operates mainly in shear. This is due to the easeof designing equipment that operates on the shear flow principle. Bycontrast, it is difficult to envisage geometry that will engender verylarge deformations in the extensional flow field. However, one mayby-pass this problem by designing a mixer in which the elongational flowfield is engendered in a series of convergent-divergent geometries,preferably with semi-quiescent zones in between.

One prior patent describing an extensional flow mixer was U.S. Pat. No.4,334,783 of Suzaka, which issued Jun. 15, 1982. The drawbacks of theSuzaka mixer are described in our aforesaid '106 patent. The mixerdescribed in our '106 patent was intended to overcome these drawbacks,and to provide a mixer having the following characteristics:

1. The mixture of two fluids is exposed to strong extensional flowfields, each followed by a semi-quiescent zone;

2. The flow fields are generated by a series of convergences anddivergences of progressively increasing intensity;

3. To reduce the pressure drop, as well as to prevent blockage of therestrictive openings, a series of holes (e.g. of the Suzaka design) arereplaced by slits;

4. The slit gaps are made adjustable.

The mixer of our '106 patent has a series of chambers separated byseveral convergent/divergent surfaces providing narrow openings betweenthe chambers. The openings are in the form of slits defined by the inneredges of protrusions formed on die members which provide theconvergent/divergent surfaces. Also, the die members subject the liquidsto gradually increasing stress, since the protrusions of the die membersare concentric and are arranged so that during mixing the liquids passradially inwards between the die members in passing from the inlet tothe outlet of the mixer. At least one of the die members is made movableto adjust the slit gap, thereby adjusting the stress level.

In the design shown in our '106 patent, the movable die member is heldat the lower end of a cylindrical block or mandrel which is slidable ina cylindrical chamber of a housing. Movement of the block, foradjustment of the gap width, is effected by rotating a wedge-shaped discbetween an end of the housing and a sloping top end of the block.Passageways for the supply of the mixed liquids to the edges of the diemembers are formed around the sides of the block, and communicate with aside inlet into the housing. This construction has been found to havetwo drawbacks.

Firstly, when using high pressures in the mixer, for example 3,000 psior 20 MPa, the liquid pressure at the side of the block adjacent theside inlet tends to tilt the block causing asymmetrical flow to theedges of the die members. Secondly, the wedge-shaped disc used to varythe slit gaps was difficult to adjust. The present invention overcomesthese problems.

Another form of the present invention combines features of the '106motionless mixer patent with some features of known dispersive mixersthat are used in association with screw extruders, particularly singlescrew extruders, to improve the mixing capability of such extruders.Such mixers generally have a housing defining a cylindrical cavity withinlet and outlet ends, and a mandrel of generally cylindrical form whichis rotatable in the cavity. The mandrel has protrusions which mayresemble screw threads, but which are interrupted by gaps, or separatedby other, discrete protrusions or indentations, so that the materialbeing mixed is not merely progressed along the cavity, as in a screwextruder, but is also caused to move through slits between the outeredges of the protrusions and the inside surface of the cavity. The sidesurfaces of the protuberances provide convergent entrances into, anddivergent exits from, the slits.

SUMMARY OF THE INVENTION

The extensional flow mixer of this invention is similar to that of our'106 patent in having:

a housing providing a cavity having an internal surface, and having aninlet into the cavity which inlet is connectable to a pressurized sourceof the liquids, the end of the housing remote from the inlet having anoutlet for the mixed liquids;

a mandrel located in the cavity;

the mandrel carrying protrusions having side surfaces which convergetowards their outer edges, the outer edges cooperating with the internalsurface of the cavity to divide the space between the protrusions andthe internal surface into a series of chambers separated by slits suchthat liquid passes successively through all the chambers and slits inmoving from the inlet to the outlet, the side surfaces providingconvergent entrances to, and divergent exits from, the slits, and theslits having cross-sectional areas which decrease in the liquid flowdirection, from an upstream chamber adjacent the inlet, to the outlet;and means for adjusting the slit gaps.

To overcome problems with asymmetrical flow of liquids into theoutermost cavity, in accordance with this invention the inlet into thehousing is at an end of the housing, rather than at the side, and themandrel has a side portion provided with helical grooves which cooperatewith an interior surface of the housing to form helical passagewaysconnecting the inlet to the upstream chamber for distributing theliquids to this chamber.

In a preferred embodiment, at least one helical groove is provided foreach 25 mm of the mandrel diameter, each groove leading from an inletend of the block mandrel to the vicinity of the upstream chamber.

In the mixer of our '106 patent, adjustment of the slit gaps wasachieved by moving the block or mandrel, which carried one series of theprotrusions. In accordance with another aspect of the present invention,the block or mandrel is stationary relative to a fixed part of thehousing, and this fixed part of the housing is connected by screwthreads to a relatively adjustable part of the housing. The relativelyadjustable part may carry protuberances which cooperate with those ofthe mandrel to define the slits.

As indicated, one form of the present invention has features in commonwith known dispersive mixers having a housing defining a cavity which isusually of generally cylindrical form, and having a mandrel rotatable inthe cavity, the mandrel having protrusions. However, the presentinvention differs from this prior art, firstly, in that the cavity isfrusto-conical having a large end at the inlet and a small end at theoutlet, and in that the protrusions are annular, and are such that saidouter edges divide the space between the protrusions and the internalsurface into a series of annular chambers separated by the slits suchthat liquid passes successively through all the chambers and slits inmoving from the inlet to the outlet. The chambers have a mean diameterwhich decreases from an outermost chamber adjacent the inlet to aninnermost chamber adjacent the outlet.

This form of the invention may also include screw thread means foradjusting the axial position of a portion of the housing relative to themandrel to alter the slit gaps. Also again, the mandrel may have,adjacent the inlet, a side portion provided with helical grooves, thegrooves forming helical passageways with an interior surface portion ofthe housing, the passageways communicating with the upstream chamber.This dynamic form of the invention, termed a dynamic extensional flowmixer (DEFM), has all four elements which constitute the fundamentalprinciples of the invention: strong, elongational flow fields,increasing in intensity in the downstream direction, and the use ofslits which are adjustable.

In this form of the invention, the fact that the mandrel rotates addsangular shear to the mixing; this is desirable as it prevents anelongated droplet from returning to a spherical shape.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings, in which;

FIG. 1 is a sectional elevation of one form of mixer of the motionlesstype,

FIG. 2 is an enlarged sectional elevation of the die members of the FIG.1 mixer,

FIG. 3 is a partial plan view of the die plates of the same mixer,

FIG. 4 is an enlarged view of a die of the same mixer, and

FIG. 5 is a sectional elevation of a dynamic extensional flow mixer(DEFM).

DETAILED DESCRIPTION

As shown in FIG. 1, the mixer has a cylindrical housing 10 with aremovable top plate 12 held on by bolts 13 which extend up from thebottom of the housing through the length of its cylindrical side wall.An adapter plate 14 is fixed to the top of the plate 12 by bolts 15. Theplates 12 and 14 have aligned axial bores 12 a and 14 a which togetherprovide an axial inlet into the end of the housing. The upper endportion of bore 14 a is threaded to receive an adapter (not shown)connected to an extruder which delivers viscous liquids at requiredpressure to the inlet. The mixer is supported by a support yoke fixed tothe mixer by support rods inserted into side bores in the housing; theposition of these side bores, which are located between the bolts 13, isindicated at 16. The housing 10 also carries a pressure sensor (notshown), which is located at 90° to the sectional plane shown in FIG. 1.

The housing 10 surrounds a cavity having a cylindrical sidewall 11 a andbeing closed at the top by the plate 12 and at the bottom by a movableend closure 18 which has a main disc portion 18 a surrounded by sidewall 18 b which seal against the cylindrical side wall of the cavity,and having a downwards extending boss 18 c provided with a central,axial outlet bore 19. This end closure 18 provides a holder for a first,movable die member 20 which provides an internal surface 11 b for theend of the cavity, and which will be further described below withreference to FIGS. 2-4. The die member has a central outlet bore 20 acommunicating with outlet 19. The end closure is adjustably supported inthe cavity by a disc-like adjusting plate 22 the outer edges of whichare provided with fine screw threads 23 mating with internal threads ofa lower end portion of the housing side wall. Side portions of the plate22 are provided with four partial bores 24, parallel to the plate axis,two of which are shown, suitable for receiving projecting parallelspigots of a tool (not shown) which can be used to rotate the plate toadjust the axial position of the movable die. The threads 23 are fineenough to allow for fine adjustment of the plate position; suitablethreads provide 2 mm of movement for each 360° of rotation of the plate.

The die member 20 is held in place in the end closure 18 by bolts 25extending up through the main disc portion 18 a of the closure intoblind threaded bores in the die member. In addition, bores 26 areprovided in the disc portion 18 a to allow the die member to be knockedout of the closure by suitable tools, when replacement is needed.

The upper end portion of the housing cavity is occupied by a block ormandrel 30, the lower end of which carries the second, fixed, die member32; again, details of this will be described in relation to FIGS. 2-4.The mandrel 30 is an integral part of the top plate 12. The die member32 is fixed to the underside of the mandrel 30 by bolts 33 having theirheads recessed into the top of the 20 plate and their lower ends engagedin blind threaded bores in the top of the die member 32.

The outer edges of the die member 32 are spaced within the insidesurfaces of the closure wall 18 b, allowing liquid to flow between theseedges. The space between these edges communicates with passagewaysformed on the outside of the mandrel 30, and which communicate with theinlet 12 a. Specifically, the outside surface of the mandrel is formedwith several, for example four, equi-spaced side-by-side spiral orhelical grooves 34, resembling a multi-start screw thread, each groovebeing of U-shaped cross section and having its outer edges close to ortouching the cylindrical interior surface 11 a of the cavity and eachforming a passageway with this surface. At their upper ends the grooveseach communicate with a radial passageway 36, several equi-spaced suchpassageways being provided, each of which communicates with a lower endportion of the inlet bore 12 a.

FIGS. 2, 3, and 4 show details of the die members 20 and 32. These carrylower and upper symmetrically opposed protrusions 20′, 32′, theseprotrusions having opposed inner edges E separated by slits. Theprotrusions have sloping side surfaces adjacent these inner edges whichprovide converging entrances into the slits and diverging exitstherefrom, and which define in part an inlet chamber C1 and twointermediate chambers C2 and C3. Typically, the slopingconvergent/divergent surfaces lie at 60° to the generally horizontalplane of the overall flow of the liquids, i.e., angle α in FIG. 4 is120°, although angles between ±15° of this preferred angle may besuitable. It will be seen that the die members provide parallel faces20″, 32″, which define intermediate portions of the chambers between thedies, these portions being more than one half and preferably more than70% the radial extent of the chambers. These provide semi-quiescentspaces. The slits are adjustable within a wide range by rotation of theadjustment plate 22, to provide convergence ratios (i.e., the ratio ofchamber depth to slit gap, or the ratio of the spacing between theparallel faces to the spaces or gaps between the inner edges E of theprotrusions) preferably of between 5:1 and 250:1. Accordingly, thetransverse dimension of the intermediate portions of the chambers, asdefined by the spacing between the parallel faces of the die members, isat least twice the slit gap. The lower die member 20 has its outlet bore20 a inwardly of chamber C3 leading to the outlet 19, while the upperdie member 32 has a central boss 32 a with a central projection shapedto divert the liquid towards the outlet.

The nature of the die members so far described is the same as that ofthe '106 patent. However, one difference over this previous patent, andwhich is illustrated in FIG. 4, is that there is a smooth transition ofthe slope from the sides of the protrusions to their inner edges E,i.e., the edges of the protrusions are rounded instead of having a sharpcorner as in the previous patent. This is intended to eliminate thepossibility of dead space or the deposition of immobile polymer at thesecorners.

In use, a blend of molten polymers enters the mixer from any pumpingdevice, e.g., an extruder or gear pump, through an adapter attached toadapter plate 14. The melt passes from the bore 12 a into the radialpassageways 36, and then into the spiral passageways formed by thegrooves 34 and the interior surface of the housing. The melt is smoothlydistributed by these passageways, and is evenly distributed around theouter edges of the die members, a result not well achieved with thedesign shown in our previous '106 patent. The melt then flows from therims of the die members towards the central outlet 19, undergoingconvergent and divergent deformation before passing out through the bore20 a and outlet 19. The gaps between the inner edges of theprotuberances can be adjusted by rotating the adjusting plate 22 usingthe tool having spigots which engage in the four holes 24 in this plate.The slit gaps can be controlled precisely within the range of from 0 to3 mm. The pressure and temperature of the melt are continuously recordedby the sensor inserted into the melt through the side wall of thehousing. The mixer can readily be mounted on a laboratory or anindustrial extruder with a throughput of up to 1,000 kg/hr.

Apart from the even distribution of liquid to the edges of the diemembers given by the axial inlet and spiral passageways, otheradvantages of this design over that of the prior '106 patent are:

1. The design is sturdy, with little deflection caused by the highpressures used. The feed is uniform around the mandrel and does notgenerate any pressure gradient that may tend to tilt the mandrel. Thisis important since with a small slit gap, say of 50 microns, adeflection of only 5 microns is significant.

2. The melt stream is partly homogenized before reaching the diemembers, the melt temperature being more uniform.

3. Pressure drop in the melt distributer system, upstream of the diemembers, is relatively low, compared to the pressure drop across thewhole mixer.

4. The machining of the mandrel, is relatively easy, compared to thatneeded to produce the special groove in the block of the former design.

5. The screw thread allows easy adjustment of the slit gap.

It may be noted that while it is desirable for both the die members 20and 32 to have ridges, it is also possible to achieve extensional flowmixing with ridges only on one die member, this member facing a flatplate.

In some cases the motionless mixer as described above may not beconvenient to install on the production line, and/or it may not provideadequate distributive mixing. For this reason the dynamic extensionalflow mixer shown in FIG. 5 has been developed. This is shown as attachedto the conventional barrel 101 and screw 102 of an extruder. The mixerincludes a housing 110 the initial or upstream portion of which is abarrel extension 114 having a flange 114′ attached to a flange of theextruder barrel 101, the upstream end of the extension defining an inlet112 a into the mixer. The extension has a retaining ring 115 whichretains an inner flange of a rotatable sleeve 112, and this sleeve hasinternal screw threads 123 holding an upstream cylindrical portion 111 aof conical mixing barrel 111 forming a downstream part of the housing110. Below the retaining ring 115 a cylindrical portion 114 b of theextension 114 engages an inner surface of the mixing barrel portion 111a via a sealing bushing 124; these portions are made non-rotatablerelative to each other so that the axial position of the mixing barrel111 can be adjusted by rotation of the sleeve 112 relative to extension114.

The mixing barrel 111 defines a cavity having a frusto-conical innersurface 111 b converging from an upper end to an outlet 119 at its smallend. A mandrel 130 has an upper end adapter 130 a connected to the screw102; an upper portion 130 b located within the cylindrical interior 114a of the extension 114; a main, lower portion 130 c located within thecavity of the mixing barrel 111; and a lower end part 130 d locatedclose to the outlet 119 from the housing.

The upper end adapter 130 a has a threaded bore by which it is attachedto the lower end of the screw 102, so that the mandrel is caused torotate with the screw. From its upper end this part decreases indiameter to meet the upper end portion 130 b, which then expands toprovide a short cylindrical part 130 b′ which fits closely within theinterior surface 114 a of the barrel extension 114, and then reduces indiameter to connect with a cylindrical connecting part at the upper endof the main mandrel portion 130 c. The upper end portion 130 b isprovided with helical grooves 134 which cooperate with the interiorsurface 114 a to form helical passageways connecting the inlet to achamber surrounding the main portion of the mandrel.

The main mandrel portion 130 c has several, for example three, threeco-axial annular protrusions 132 which are of decreasing diameter so aseach to have an outer edge close to the conical internal surface 111 bof mixing barrel 111. The protrusions divide the space between theinternal surface 111 b and the mandrel into co-axial chambers C1, C2, C3and C4. The spacing between the outer edges of the protrusions and theinternal surface 111 b is adjustable by rotation of the sleeve 112 toshift the axial position of the barrel 111. Since a small amount ofaxial movement of the barrel results in much smaller radial changes inthe slit gaps, very fine adjustment is possible. Usually gaps of 0 to 4mm. will be used. Typically, the barrel sides are inclined at between 10and 15° to the barrel axis, and an amount of axial movement of 1 mmchanges the slit gaps by about 170 to 270 micrometers.

While the drawing shows protuberances with apparently sharp outer edges,these will preferably be rounded, as shown in FIG. 4 for the firstembodiment.

The downstream end portion 130 d of the mandrel has an upstream flankincreasing in diameter from chamber C4 to a short cylindrical section130 d′, and a downstream flank decreasing in diameter with a slopeslightly larger than that of the housing barrel 111. The two flanks arejoined by grooves 140 which provide passageways between the last chamberC4 and the outlet 119.

In operation, this mixer provides both dispersive and distributivemixing, the former caused by the convergent/divergent flow of the liquidthrough gaps between the rotating members 132 and the extended barrelsurface 111 b, i.e., the gaps separating chambers C1, C2, C3, and C4.The distributive mixing is ascertained by mainly shear flow of the meltthrough the grooves of members 130 b and 130 d. The pressure of liquidsentering the inlet end of the mixer, via the grooves 134, causes theliquids to pass successively through the chambers C1 to C4, passingthough all the slits in moving from the inlet to the outlet.Accordingly, the mixer works, in this sense, similar to that of thefirst embodiment. Also, as in the first embodiment, the length, as wellas the areas, of the slits decreases as the liquids pass through themixer, so that they are subjected to increasing extensional stress.Here, however, the mandrel is rotated by its connection to the extruderscrew, and accordingly there are also shear forces between the rotatingparts of the mandrel and the internal surface of the barrel, especiallywhere the liquids are close to the protuberances. The amount of shear ishowever relatively minor, and not such as to cause degradation of mixedpolymers.

The FIG. 5 embodiment is easily scaled up and can be adapted for morestages. Unlike the first embodiment, the number of slits can beincreased without undue increase in the diameter. Helical grooves can beprovided not only in inlet member 130 b but also in outlet member 130 d.

The form of the mandrel shown in FIG. 5 also provides semi-quiescentzones, as in the first embodiment, where the liquid body is neitherbeing strongly contracted or expanded. The shape and size of thechambers C1 to C4 can be optimised using the finite element flowmodelling for the melt of typical viscoelastic characteristics.

The FIG. 5 embodiment does not need to be attached to a single-screwextruder, as shown, but may also be incorporated in a design using twinscrews. Also, the mixer can be used as a stand-alone unit, having, ifdesired, its own independent power source, as well as an internal mixerin blow molding and injection molding machines. While the mixer willprovide extensional flow mixing when stationary, some rotation isdesirable for the angular shear it provides. The rotational speedhowever may be low. The mixer is not limited to polymers, and can beused in mixing foodstuffs, homogenizing milk, and preparation ofemulsions.

We claim:
 1. An extensional flow mixer suitable for high viscosityliquids such as plastic materials, comprising: a housing (10) with acylindrical cavity having a side wall (11); an axial inlet (12 a) intosaid cavity at one end of said housing and connectable to a pressurizedsource of the liquids; an outlet (19) for the mixed liquids leading fromthe cavity, said outlet being at the center of an outlet end of thehousing opposite said one end; a first die member (20) in said cavity atsaid outlet end of said housing; said first die member carrying annularconcentric protrusions (20′) surrounding a central aperture (20 a) whichcommunicates with the outlet; a mandrel (30) located in said cavity, asecond die member (32) fixed to said mandrel and carrying annular,concentric protrusions (32′), the protrusions of the first die memberhaving inner edges symmetrically opposed inner edges of the second diemember, and said protrusions having sloping side surfaces to divide thespace between the die members into a series of annular chambers (C1,C2,. . . ) separated by annular slits between said inner edges, with saidsloping side surfaces providing convergent entrances to, and divergentexits from, the slits; means (22,23) for adjusting the position of oneof said die members in the housing to alter the slit gaps; characterizedin that said mandrel has sides provided with helical grooves (34) andhas radial passageways (36) connecting said helical grooves to saidinlet, said grooves forming helical passageways with said side wall(11), said passageways communicating with outer edges of said diemembers for distributing the liquids around the edges of the diemembers.
 2. A mixer according to claim 1, wherein at least four of saidhelical grooves (34) are provided.
 3. A mixer according to claim 1,wherein said first die member (20) is mounted on holder means (18)having screw threaded engagement (23) with a portion of the housing,said holder means being rotatable so as to be movable axially within thehousing and so as to adjust the slit gaps.